Recently, electric apparatuses, such as microwave ovens, having an electronic control device in the form of a combination of a microcomputer and a keyboard have been put to practical use. Concurrently therewith, buzzers and other sound producing devices are being used in large quantities.
The sound producing property of such sound producing device depends largely on the arrangement of the sound producer and resonance box and also varies greatly with the way they are assembled. Thus, in order to stabilize the sound producing property, it has been necessary to arrange the sound producer and resonance box so that the sound producing property does not vary with the way they are assembled.
For example, a conventional buzzer 1 of this type used in a microwave oven, as shown in FIGS. 1-5, comprises a resonance case 2, a sound producer 3, and input lead wires 4 and 5 connected to the electrode plates of the sound producer 3 to obtain electric signals from the outside. As for the sound producing principle of this type of sound producing devices, there are two types, the separate excitation type wherein both ON- and OFF-signals are applied to the lead wires 4 and 5, and the self-excitation type wherein the piezoelectric property of a ceramic sound producer is utilized to apply only ON-signals thereto to cause vibration. The conventional example is of the separate excitation type.
The sound producer 3 is built up of a metal electrode plate 6 and a ceramic electrode plate 7 which are bound together by an adhesive agent 8. The peripheral edge of the metal electrode plate 6 of the sound producer 3 is fixed to the peripheral edge of an opening 9 at one end of the resonance case 2 by an adhesive agent 10, as shown in FIG. 5. In this condition, if an oscillation waveform is applied to the input lead wires 4 and 5, the ceramic electrode plate 7 of the sound producer 3 is deflected according to the frequency of the ocillation wave and produces a sound. The sound pressure is determined when the resonance frequency dependent on the volume of the resonance case 2 and on the inner diameter of a sound release hole 12 formed in the other end surface 11 of the resonance case 2 coincides with the frequency of the oscillation waveform applied from the outside. In addition, the resonance frequency f of the resonance case 2 is ##EQU1## where f is the resonance frequency; C is the velocity of sound; A is the radius of the sound release hole 12; D is the support diameter of the sound producer 3; H is the depth of the resonance case 2; K is a constant; and L is the depth of the sound release hole 12.
This conventional sound producing device has the following drawbacks which have been serious problems.
(1) If the amount of the adhesive agent 10 fixing the sound producer 3 and the resonance case 2 to each other is too small, there is formed a clearance between the resonance case 2 and the sound producer 3, producing a dissonance which disturbs the sound. Further, if the amount of the adhesive agent 10 is too large, as shown in FIG. 5, the adhesive agent 10 intrudes into the resonance case 2, changing the support diameter D of the sound producer 3, so that, as is clear from equation 1, the resonance frequency f changes and the sound pressure decreases. Thus, what is required is uniform application of a proper amount, but this is difficult to attain and hence the performance has been unstable.
(2) As shown in FIG. 6, a sound producing device 13 comprises a control-purpose printed circuit board 14, the conventional buzzer 1 electrically connected on said board, and oscillation circuit parts 15 for vibrating said buzzer 1 disposed around the latter. However, a large installation space is required, increasing the size of the control-purpose printed circuit board 14.